August 2012

Spider mites love this hot, dry weather

Spider mites on coneflowers. (C) Jo Ellen Meyers Sharp

Those specks of rusty gold on the leaves of coneflowers, tomatoes, dahlias, arborvitae, spruce and dozens of other plants in the garden are one more thing to blame on the excessive heat and drought.

The stippling is caused by spider mites, creatures that aren’t really spiders, but are closely related. Spider mites flourish in hot, dry weather. The weather also suppresses a natural fungus that helps control the mites, allowing the bugs to remain unchecked.

Spider mites puncture the cells of leaves and suck out the chlorophyll, leaving small rusty, gold or creamy spots. Each mite can puncture 100 cells an hour and there may be thousands of mites on a plant. There are several different mites and they tend to be fairly specific to certain types of plants. For instance, the mites on a dahlia are not the same as those on an arborvitae.

Usually you see the damage before you see the tiny yellow, green or brown mites. They work on the undersides of leaves and branches, forming visible webs on plants to help them move about. To test for mites, shake a leaf or branch above a white piece of paper. If dots that move appear on the paper, the plant likely has spider mites.

Fortunately the mite damage is not usually deadly. The first line of defense is a strong spray of water from the hose to wash off the mites from plants. Remove any visible webs. Insecticidal soap, a natural insecticide available at garden centers, also can be used to control heavy infestations of spider mites. Horticulture oils smother spider mites and frequently are recommended for evergreens. Horticulture oils are temperature sensitive, so make sure the one you use is seasonally appropriate. Make sure to spray the undersides of leaves. Always read and follow the label directions.

For more information about this pest, download Purdue University’s Spider Mites on Ornamentals or Ohio State University’s Spider Mites and Their Control.



You Can Grow That! August 2012: Hydrangeas

Limelight (Hydrangea paniculata) flowers turn pink as they age. Photo courtesy Proven Winners/ColorChoice

One of the showier shrubs in the garden is one that offers year-round interest. Hydrangeas, with their moptop and lacecap flowers, grab centerstage in the summer garden. And, they hold a place for two more acts as their flowers dry to autumn hues, then winter whites.

There are two basic types of hydrangeas (Hydrangea), those that bloom on year-old growth and those that bloom on current season growth. Some newer cultivars bloom on old and new wood.

The big-leaf hydrangea (H.  macrophylla) forms its flower buds in late summer and fall. In the upper Midwest and other cold climates, freezing temperatures in late spring frequently zap these buds, causing the plant to grow leaves, but no blooms.

Let's Dance Moonlight big leaf hydrangea blooms on new and old growth. Photo courtesy

You can reduce this threat by siting a big leaf hydrangea where it is buffered from drying winds and freezing temperatures. Many northern gardeners protect it by making a sleeve to fit around the plant and filling it with leaves in fall. The leaf-stuffed sleeve stays on the plant until spring, when there’s no danger of a hard freeze. The sleeve can be made of cloth, such as burlap, or it can be a plastic trash can with the bottom cut out.

Most big-leaf hydrangea is better suited for the more moderate climates in the West, South and Southeast. H. serrata is another popular hydrangea that blooms on year-old growth and can be susceptible to flower bud damage from cold temperatures. If you need to prune these hydrangeas do so within a month after it blooms.

Blushing Bride in Bailey Nurseries' Endless Summer Hydrangea series, has white flowers with a blush of pink. Photo courtesy Bailey Nurseries

Several breeders have introduced big leaf hydrangeas that bloom on current season and year-old growth. The Endless Summer, Forever and Ever and Let’s Dance series of plants are in this group. The color of the flowers on these hydrangeas depends on the soil. Acidic soil produces flowers in the blue range and hydrangeas planted in alkaline soil have blooms in the pink palette. You can add alumninum sulphate to hydrangeas planted in alkaline soil to turn them blue and lime to those planted in acidic soil to turn them pink.

Although the native oakleaf hydrangea (H. quercifolia) blooms on year-old growth, it does so later in the season so its flower buds are less likely to be damaged by freezing temperatures in spring, making it quite reliable in northern gardens. Oakleaf hydrangeas have cone-shaped flowers that turn pinkish as they age, persisting on the plant well into winter. The leaves on this hydrangea turn a leathery, wine-red in fall and stay on the plant through winter and into spring. The bark flakes off, called exfoliating, to reveal a cinnamon color. Some cultivars to consider: ‘Alice’, ‘Pee Wee’, ‘Snowflake’, ‘Snow Queen’ and ‘Sikes (Sykes) Dwarf’.

The colors have begun to change on the oakleaf hydrangea. (C) Jo Ellen Meyers Sharp

The most reliable selections for cold climates are the native smooth-leaf hydrangea (H. arborescens) and H. paniculata, commonly referred to as peegee hydrangea, from Asia. Both of these hydrangeas bloom on current season growth, which means you can prune them in spring without cutting of summer’s flowers. The flower colors on these hydrangeas cannot be altered by soil additives.

Popular smooth-leaf hydrangeas are the mop-top ‘Annabelle’ and the lace-cap ‘White Dome.’ ‘Invincibelle Spirit’ and ‘Annabella’ (H. arborescens) are breeding breakthroughs because they bloom with pink moptop flowers.

A tremendous amount of breeding is going on with the peegee hydrangeas. These form cone-shaped flowers, called panicles, that take on various hues of white, green or pink.

Breeders are working on plants with flowers that turn pink faster or bloom in pink. Some of the best H. paniculata are: ‘Tardiva’, ‘Pinky Winky’, ‘Quick Fire’, ‘Strawberry Vanilla’, ‘Limelight’ and ‘Little Lime’.

Plant hydrangeas in shade to part sun. They tolerate full sun, but will need supplemental watering. All hydrangeas do best in soil rich in organic matter and when given a good, periodic soaking during hot, dry spells. The native smooth-leaf hydrangea can handle fairly wet to dry spots.

Apply about an inch of compost, rotted manure or other organic matter to the soil around hydrangeas in spring as they start to develop their leaves and in fall. If using a soil additive or hydrangea fertilizer, always read and follow the label directions.

Annabelle (Hydrangea arborescens) is a long-blooming cultivar of a native plant. (C) Jo Ellen Meyers Sharp

You Can Grow That! is the meme created by C.L. Fornari at Whole Life Gardening. The 4th of every month, several bloggers post columns that encourage gardeners to grow plants.